GREEN CHEMISTRY: MICROWAVE SYNTHESIS OF IONIC LIQUIDS
AND THEIR USE AS MEDIUM IN DIELS-ALDER REACTION
Anthony J. Lilienthal, Arthur J. Koudelka, Benjamin C. Haenni, and Gholam A. Mirafzal (Mentor)
College of Arts and Sciences, Department of Chemistry, Drake University
Abstract
The effectiveness of the substitution of ionic liquids for traditional aromatic solvents in the DielsAlder Reaction was investigated. Three methylimidazolium based ionic liquids – 1-butyl-3methylimdazolium bromide, 1-butyl-3-methylimidazolium iodide and 1-methyl-3-(p-toluene
sulfonyl)imidazolium chloride – were synthesized in the microwave and were then used as
solvents/catalysts in the Diels-Alder reaction between maleic anhydride and 1,3-cyclohexadiene.
The results presented here illustrate how ionic liquids are effective and recyclable solvents for the
Diels-Alder reaction.
Recyclability Tests
The aqueous layer, containing [BMIM][Br], was extracted from the Diels-Alder reaction mixture and was
placed into a rotary evaporator with a 75°C hot water bath until only the viscous ionic liquid remained
(approximately 2 hours). The ionic liquid ([BMIM][Br]) was then reused in the Diels-Alder Reaction
described. TLC and FT-IR were performed, confirming that the ionic liquid was not changed during the
reaction.
Results and Discussion
Introduction
Diels-Alder Reaction with [PtolsMIM][Cl]
The effectiveness of 1-methy-3-(p-toluenesulfonone)imidazolium chloride ([PtolsMIM][Cl]) and
freshly synthesized [BMIM][Br] and [BMIM][I] was compared. The % conversion of the maleic
anhydride from the [PtolsMIM][Cl] solvated reaction was 65.819% while the % conversion for
[BMIM][Br] and [BMIM][I] were 83.854% and 97.887% , respectively (Figure 1). The GC spectra
of the organic layers from each of the Diels-Alder reactions show corresponds with the previous
results from the previous presentation, where maleic acid (converted from excess maleic anhydride
during the extraction) has a retention time of 9.324 minutes while the product has a retention time of
9.741 minutes. Even with it’s decrease % conversion, since the ionic liquid-solvated Diels-Alder
reaction was performed at room temperature, it is still a more effective solvent than the volatile
molecular solvents.
Ionic Liquid Synthesis
Most ionic liquid synthesis procedures require too much time to be used for educational purposes;
Ionic Liquids have been termed “Designer Solvents”1 due to their variable physical properties.
3 It
2
however,
Microwave-Enhanced
ionic
liquid
synthesis
reduces
the
time
required
from
hours
to
minutes.
Due to their low volatility and recyclability, they are also considered Green Solvents. Ionic liquids
Recyclability Tests
was
demonstrated
that
[BMIM][Br]
and
[BMIM][I]
could
be
synthesized
in
as
little
as
2
minutes.
could offer students the opportunity to perform many chemical synthesis reactions without using
Since, in some situations, it is more efficient for the ionic liquids to be prepared before the students
Similarly,
[MPtolsIM][Cl]
was
synthesized
via
2
minutes
of
microwave
radiation
(Scheme
1).
harsh, volatile organic solvents. The synthesis of a new ionic liquid and its effectiveness as a
attend the lab, a recyclable solvent would allow more ionic liquid to be conserved. It was concluded
solvent for the Diels-Alder reaction between 1,3-cyclohexadiene and maleic anhydride is herein
Scheme 1: The General Equation of Microwave-Enhanced
through TLC and FT-IR analysis that the removal of water from aqueous [BMIM][Br] is sufficient to
Synthesis
of
methylimidazolium
halide
ionic
liquids.
described. In addition, the recyclability of another ionic liquid is also described.
recycle bromide ionic liquid. [BMIM][Br] was able to solvate three repetitions of the Diels-Alder
a) [BMIM][Br]
R = Butyl
X = bromide
reactions (Figure 2) using this recyclability procedure; moreover, the percent maleic anhydride
b) [BMIM][I]
R = Butyl
X = iodide
converted
during
the
Diels-Alder
reaction
solvated
by
[BMIM][Br]
actually
increased
as
the
c) [MPtolsIM][Cl] R = P-toluenesulfonyl X = chloride
Materials and Methods
repetitions were performed. It is predicted that [BMIM][I] will have similar recyclability properties,
Maleic anhydride, 1,3-Cyclohexadiene, 1-iodobutane, 1-bromobutane, 1-methylimidazole, and psince the dehydration of aqueous layers containing [BMIM][I] also purified the compound and
toluenesulfonyl chloride were all purchased from the Sigma Aldrich company and were used
[BMIM][I] has similar properties to [BMIM][Br]; however, more extensive experiments must be
without any further purification.
performed in order more accurately describe the recyclability butylmethylimidazolium halide ionic
liquids as solvents for the Diels-Alder reaction.
FT-IR were performed neat on a Nicolet 510P Spectrometer. FID-GC were performed on an
H6890 series instrument, equipped with an HP-5 (Crosslinked 5% PH ME Siloxane) column with
% Maleic Anhydride Converted
30m x 0.32 mm x 0.25 μm film thickness. During the analyses of the Diels-Alder Reaction
Ionic Liquid DA Reaction 1 DA Reaction 2 DA Reaction 3
products, the initial column temperature was 60°C (1 minute), which was then increased 10°C/min.
to 250°C (1 minute).
[BMIM][Br]
83.854
97.482
98.885
Experimental Procedure
By comparing the IR spectra of the ionic liquids [BMIM][Br] and [MPtolsIM][Cl] with the 1Ionic Liquid Synthesis (Scheme 1)
methylimidazole spectra, it was concluded that the diethyl ether washes were insufficient to remove the
Butylmethylimidazoluim Halide Ionic Liquid Synthesis:
remaining 1-methylimidazole, since many of the 1-methylimidazole peaks were also present in the
15 mmoles of 1-bromobutane (or 1-iodobutane) and 15 mmoles of 1-methylimidizole (MIM) were [MPtolsIM][Cl] spectra; however, indications of the ionic liquid product are still present. For example,
added to a small beaker and heated for 2 minutes (8 x 15 s.) at 10 % power in a SHARP Carousel the [MPtolsIM][Cl] spectra has peaks that are indicative of aromatic C= C bonds (1581.5 and 1455.5 cmMicrowave. The product was washed twice with diethyl ether to remove any possible remaining
1). These sharp peaks are observed on the [MPtolsIM][Cl] spectra, but not in the 1-methylimidazole
reactants.
spectra and could not have indicated a contaminant p-toluenesulfonyl chloride. P-toluenesulfonyl
chloride’s role as the limiting reagent suggests that all the compound had reacted, and it is quite soluble in
Methyl(p-toluenesulfonyl)imidazolium Halide Ionic Liquid Synthesis:
diethyl ether. Another indicator of [MPtolsIM][Cl] is the broad peak at 1200 cm-1, which may suggest the
5 mmoles of p-toluenesulfonyl chloride and 20 mmoles 1-methylimidazole (MIM) were mixed (20 simultaneous presence of sulfnyl S=O bond and aromatic nitrogen, both of which have characteristic
s.) into a slurry in a small beaker. The slurry was heated for 2 minutes (8 x 15 s.) at 10% power in peaks near 1180 cm-1 .3,4 Also, the S=O peak from the p-toluenesufonyl chloride peak is broad, which
a SHARP Carousel Microwave. The product was washed three times (4 mL each) with diethyl
would account for the width of the peak on the [MPtolsIM][Cl] spectra.
ether to remove any possible remaining reactants.
Diels-Alder Reaction
1.2 mmoles of maleic anhydride and a magnetic stirbar were added to a 10 mL round bottomed
flask. To this 6.0 mmoles of cyclohexadiene was added. Immediately following the addition of the
cyclohexadiene, 1.00 mL of the ionic liquid (1-butyl-3-methylimdiazolium bromide ([BMIM][Br]),
1-butyl-3-methylimidazolium iodide ([BMIM][I]), or 1-methyl-3-(p-toluenesulfonyl)imidazolium
chloride ([MPtsIM][Cl])) was added and the reaction was mixed at room temperature for 45
minutes. The Diels-Alder product was then separated from the ionic liquid by the addition of equal
volumes diethyl ether and water. This organic layer was analyzed via FID-GC to determine the
%conversion of maleic anhydride into the Diels-Alder Product (Scheme 2).
Scheme 2
% Maleic
Ionic
Anhydride
Liquid
Converted
[BMIM][Br]
83.854
[BMIM][I]
97.887
[MPtolsIM]
65.819
[Cl]
Figure 1
Conclusion
Figure 2
Ionic liquids offer educators the opportunity to teach students without using hazardous, volatile
organic solvents. Three methylimidazolium halide ionic liquids were successfully synthesized in 2
minutes via microwave-enhanced synthesis; however, the 1-methyl-3-(p-toluenesulfonyl)imidazolium
chloride was unsuccessfully purified, which, at least partially, accounted for its low maleic anhydride
conversion rate during the Diels-Alder reaction. Even with its decreased conversion rate,
[PtolsMIM][Cl] was still a more effective solvent for the Diels-Alder reaction, since classic solvents
would require hours at high temperature for the reaction to go to completion. Finally, the
butylmethylimidazolium bromide ionic liquid were shown to be easily recyclable; multiple DielsAlder reactions could be performed using the same ionic liquid, as long as the aqueous layer was
evaporated prior to the repetition of the experiment.
Future Work
We are currently experimenting to replace 1,3-cyclohexadiene with less volatile dienes, such as
anthracene. Also, the effect of ionic liquid solvation in other reactions will be investigated. Further
tests will be performed on the [PtolsMIM][Cl] to further purify it. After which, effectiveness in the
Diels-Alder reaction will be reassessed.
References
Hon Mau et al. “Ionic Liquids: Just Molten Salts After All?” Molecules. 2009, 14 (7) 2521-34.
2Varma, Rajender S. "Expeditious Synthesis of Ionic Liquids using Ultrasound and Microwave Irradiation," Ionic Liquids as Green Solvents: Progress and
Prospects. Ed. Robin D. Rogers and Kenneth R. Seddon. Boston: American Chemical Society. (2003) 82-92.
3 Abdel-Wahab. M. F. et al, “Infrared Study of New Sulfonylurea Derivatives and Sulfonamides.” Analytical Chemistry. 1966, 38 (3) 508.
4Chatwal, Gurdeep R., “Application of Infrared Spectroscopy to Organic Compounds.” Spectroscopy. Himalaya Publishing House. (2009) 2.70.
1Yau,